A description will be given of a suction air negative pressure detecting apparatus of a throttle body in accordance with a prior art with reference to FIGS. 8 and 9.
FIG. 8 is a top plan view showing a state in which a pressure sensor is attached to a throttle body.
FIG. 9 is a vertical sectional view of a main portion along a line A—A in FIG. 8.
Reference numeral 30 denotes a throttle body in which a suction air passage 31 is provided so as to penetrate sideways. The suction air passage 31 is opened and closed by a butterfly type throttle valve 33 fixed by screw to a throttle valve shaft 32 rotatably supported to the throttle body 30.
Reference symbol S denotes a pressure sensor replacing a pressure change within the suction air passage 31 to a voltage change so as to detect. The pressure sensor S is constituted by a pressure conversion element such as a silicon diaphragm or the like, and a hybrid IC amplifying an output signal of the pressure conversion element, and a suction air negative pressure within the suction air passage 31 is introduced to one side of the silicone diaphragm via a negative pressure introduction hole Sa.
The pressure sensor S mentioned above is shown in Japanese Unexamined Patent Publication No. 8-261080.
The pressure sensor S is fixedly arranged within a sensor case 34 by a potting member P formed by a synthetic resin material, and the sensor case 34 is open toward a lower side.
The sensor case 34 is provided with a cylindrical wall portion 34a, a cylinder portion 34b protruding further toward an upper side from an upper bottom portion 34d and a connector portion 34c in which a terminal 35 is arranged in a protruding manner, and the negative pressure introduction hole Sa of the pressure sensor S is arranged so as to be open toward a lower side while facing to an inner side of the cylindrical wall portion 34a. 
Further, a terminal Sb in a sensor side extending to a side portion from the pressure sensor S is electrically connected to an inner end of the terminal 35 by soldering or the like.
On the other hand, a cylinder boss portion 30a to which the cylinder wall portion 34a of the sensor case is rotatably inserted is formed in the throttle body 30 so as to protrude toward an upper side, and a downstream side of a negative pressure introduction hole 35 for the sensor is open to a concave portion in an upper end of the cylinder boss portion 30a. 
An upstream side of the sensor negative pressure introduction hole 35 is open toward the suction air passage 31 in a downstream side of a throttle valve 33.
In this case, a suction air passage 31a in the downstream side of the throttle valve communicates with an engine (not shown), and a suction air passage 31b in an upstream side of the throttle valve 33 communicates with an air cleaner (not shown).
Further, the pressure sensor S attached to the sensor case 34 mentioned above is fixed to the throttle body 30 in the following manner.
First, an inner periphery of the cylinder wall portion 34a of the sensor case 34 is arranged so as to be inserted to an outer periphery of the cylinder boss portion 30a of the throttle body 30. At this time, an O-ring 36 is provided in a compression manner with respect to an outer periphery of the inner peripheral boss portion 30a of the cylinder wall portion 34a. 
Accordingly, the inner periphery of the cylinder wall portion 34a and the outer periphery of the cylinder boss portion 30a are held in an airtight manner by the O-ring 36.
Secondly, the sensor case 34 is screwed and fixed to the throttle body 30 by a pressure plate 37. The pressure plate 37 is formed in a flat plate shape, and a case insertion hole 37a inserting the cylinder portion 34b of the sensor case 34 and a screw insertion hole 37b for inserting a screw are provided in the pressure plate 37, (in which the sensor insertion hole 37a is formed in a segmental circular shape).
Further, the pressure plate 37 is arranged on the upper bottom portion 34d of the sensor case 34, the cylinder portion 34b of the sensor case 34 is inserted to the case insertion hole 37a, the pressure plate 37 is arranged on an upper end surface of an attachment boss 30b protruding to an upper side of the throttle body 30, and the screw 38 is screwed into the attachment boss 30b via the screw insertion hole 37b of the pressure plate 37 in this state. Accordingly, the sensor case 34 provided with the pressure sensor S is pressure fixed toward the throttle body 30 by the pressure plate 37.
Therefore, a negative pressure generated within the intake passage 31a is introduced into the pressure sensor S via the sensor negative pressure introduction hole 35 and the negative pressure introduction hole Sa, and the pressure sensor S outputs a voltage signal corresponding to the negative pressure toward an external portion via a sensor side terminal Sb and a terminal 35.